1. Field of the Invention
The invention is related to the field of pH electrodes, and in particular, to a low-slope pH electrode.
2. Statement of the Problem
A measure of the ionic concentration of a fluid is desirable in many situations, including testing of fluids in manufacturing settings, for pharmaceutical production, food processing and/or food quality, water quality testing, etc. Measurement of an ionic concentration or activity can indicate completion of a reaction, indicate fractions of components, etc. One measure can comprise a measure of pH, which comprises a measure of acidity of the fluid being tested. The pH measurement can indicate the acidic or basic condition or concentration of the fluid.
A pH measurement comprises a measurement of hydrogen ions in a solution, expressed as a logarithmic number between about zero and fourteen (sometimes extending into negative numbers for exceedingly acidic solutions). On the pH scale, a very acidic solution has a low pH value, such as zero or one, corresponding to a large concentration of hydrogen ions (H+). In contrast, a very basic solution has a high pH value, corresponding to a very small number of hydrogen ions (or to a correspondingly large number of OH− ions). A neutral solution, such as substantially pure water, has a pH value of about seven.
A pH electrode is an electrode that generates a voltage potential in relation to the pH level of a solution. Sensitivity and accuracy are obviously important characteristics of a pH electrode. Glass electrodes that are used as pH sensors are designed to respond to the pH of a solution, with the measured electrical potential (mV) being plotted on the Y-axis, and pH units being plotted on the X-axis of a graph.
FIG. 1 shows a prior art pH electrode. The prior art pH electrode is a glass tube with an ion sensitive glass bulb at one end and the tube further includes an electrolyte and internal electrode element. The prior art pH electrode is typically constructed of two types of glass. The stem of the electrode is a non-conductive, non-responsive glass, while the tip, which is most often bubble-shaped, is a specially formulated pH sensitive lithium ion-conductive glass consisting of the oxides of silica, lithium, and other elements. The structure of the pH glass allows lithium ions to be exchanged with hydrogen ions in aqueous solutions, forming a hydrogen ion responsive layer on the outside of the bulb. A millivolt potential is created across the interface between the pH-sensitive glass and the external aqueous solution. The internal electrolyte interacts with the ion sensitive glass, forming an internal potential.
The magnitude of the potential on the outer surface is dependent on the pH value of the solution being measured. The difference between the external and internal surface voltage potentials (Vexternal minus Vinternal) that are created at the outer and inner hydrated layers of the pH glass can be measured through electro-chemical contact by an internal electrode at least partially immersed in an internal electrolyte solution. Since the internal solution of the glass electrode is held at a constant pH, the measured potential difference is dependent only upon the pH value of the external solution being measured.
A reference electrode is often included in a separate chamber and solution that are also in ionic communication with the fluid being tested. A voltage potential between the two electrodes is thereby formed, similar to a battery. The voltage potential that is developed between the electrodes is directly related to the hydrogen ion concentration of the solution. The reference electrode provides a stable potential against which the measuring electrode can be compared. The voltage potential can be processed according to a table, formula, or other algorithm to arrive at an ionic concentration measurement, such as a pH value, for example.